Has Your SD Card Mysteriously Shrunk? A Mac User's Guide to Restoring Its Full Size
Have you ever taken an SD card you used for a project, like setting up a Raspberry Pi, and popped it back into your Mac, only to find it's a fraction of its original size? That 64GB card might suddenly appear as a measly 60MB drive. It’s a frustrating experience, but don't worry, your card isn't broken! This guide will walk you through why this happens and how to fix it using a powerful tool already on your Mac.
(A picture showing a "Get Info" window on a Mac for an SD card, displaying a very small capacity like 64MB, with the actual 32GB or 64GB card next to it for comparison.)
The "Why": Understanding the Shrinkage
The main reason this happens is due to how operating systems like Raspbian (now Raspberry Pi OS) are prepared. When you "burn" or "flash" an image like this to an SD card, the process splits the card into multiple sections, known as partitions.[1] One of these is a small boot partition that your Mac can easily read, and another is a much larger Linux partition that Mac's operating system doesn't automatically show.[1]
So, when you plug the card into your Mac, it only mounts and displays the small, readable boot partition, creating the illusion that your card has lost most of its storage space.[2] Trying to reformat it using the standard Disk Utility app often fails because it, too, can only see and interact with that small partition.[1]
The Fix: A Fresh Start with Terminal
To fix this, we need to wipe all the partitions at once and create a new, single one. The best tool for this job on a Mac is the Terminal. If you've never used it before, don't be intimidated! It's a simple, text-based way to give your computer direct commands. We'll go step-by-step.
Disclaimer: This process will completely erase all data on the SD card. Before you begin, make sure there is nothing on the card that you need to save. Also, it is crucial to follow the steps carefully to avoid erasing the wrong disk.
Step 1: Open the Terminal
First, we need to open the Terminal application. The easiest way is with Spotlight Search.
- Press Command (⌘) + Spacebar on your keyboard to open the Spotlight search bar.
- Start typing "Terminal".
- When "Terminal.app" appears, double-click it to open.[3][4][5]
You will be greeted with a simple window with a text prompt. This is where we'll type our commands.
Step 2: Find Your SD Card's ID
Now, we need to correctly identify which disk is your SD card. This is the most important step.
- Insert your SD card into your Mac.
- In the Terminal window, type the following command and press Enter:
diskutil list
This command will print a list of all the disks connected to your computer. Look for a disk labeled as (external, physical). Match the size shown with the size of your SD card. The identifier we need is on the left, formatted as /dev/disk followed by a number (e.g., /dev/disk2, /dev/disk3, etc.).[1][6][7]
(A screenshot of the diskutil list output, with a red box drawn around the external SD card's information, highlighting the identifier like /dev/disk2.)
Warning: Your Mac's main hard drive is usually disk0 or disk1. Be absolutely sure you have identified the correct external disk before proceeding. In our example, the SD card is /dev/disk2.
Step 3: Erase and Reformat the SD Card
Once you have the correct disk identifier, you're ready to erase the card.
- Type the following command into the Terminal, but replace /dev/diskX with the identifier for your SD card that you found in Step 2.
sudo diskutil eraseDisk FAT32 SDCARD MBRFormat /dev/diskX
Let's break down that command:
- sudo: A command that lets you run things as a "super user," with the necessary permissions to perform this action.[8]
- diskutil: The name of the command-line disk utility.[9]
- eraseDisk: The specific action you want to perform.[8][9]
- FAT32: The new file system for the card. This is a highly compatible format that works with most cameras, computers, and other devices.[6]
- SDCARD: The new name for the SD card after it's formatted. You can change this to whatever you like (e.g., "RASPBERRY" or "GoPro").
- MBRFormat: This specifies the partition scheme as "Master Boot Record," which is a reliable and widely compatible choice.[10]
- /dev/diskX: The target disk to be erased. This must be the correct identifier for your SD card.
After typing the command, press Enter. The Terminal will ask for your password. This is your Mac's login password.
- Type your password and press Enter. Note that for security reasons, you will not see the cursor move or any characters appear as you type. This is normal.
The Terminal will then show you the progress as it unmounts the disk, erases it, and creates the new partition and format.[11]
(A screenshot of the successful output in the Terminal after the command has finished, showing the steps like "Unmounting disk," "Erasing," and "Finished.")
Step 4: All Done!
That's it! Your SD card should now reappear on your desktop and in Finder with its original, full capacity restored. It's now a clean, single-volume card, ready for your next project.
(A final screenshot showing a "Get Info" window for the same SD card, but this time displaying its full, correct capacity, e.g., 32GB.)
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